Lasers are used today for processing a plurality of technical materials such as, e.g., metals, plastics and also ceramics. The material to be processed melts or evaporates due to the high energy, which is concentrated in a very small focal spot and is made available in the form of light which is normally coherent and monochromatic. The fact that the laser beam as such is not subject to any wear and that a direct material contact between tool and workpiece is eliminated is especially advantageous in the use of lasers as tools.
In order to obtain the best possible processing result, it is necessary that the laser beam is optimally focused on the workpiece to be processed. Otherwise, the spread light energy is distributed onto too great an area, so that the temperatures achieved in the irradiation are possibly insufficient for an effective processing. Moreover, it is frequently desirable that the area processed by the tool and in the form of a cut edge turns out to be as narrow as possible, which is also achieved by an optimally focused laser beam.
In order to adjust the smallest possible focal spot the lenses present in laser systems have appropriate adjustment possibilities for the focal distance. Even though these lenses can possibly also be actuated automatically, it is nevertheless necessary to perform the desired adjustment of the focal distance manually to the extent that the result is sought from a series of tests with different settings of the lens at which result the focal spot is the smallest. The parameters belonging to this result are subsequently passed on to the lens of the laser system.
To this end a plurality of tests with different focusing adjustments is usually carried out. Such a test series is also called a focus series. The evaluation of the results take place manually by an operator. Since the focal spots to be examined are frequently very small by nature, this examination takes place in the normal case with the additional aid of auxiliary optical means such as, for example, magnifying glasses or microscopes.
This manner of procedure entails a number of disadvantages. At first, the quality of the results is a function of the experience of the operator who carries out the examination and the selection of the optimal focal spot. Furthermore, the carrying out of the tests and the subsequent evaluation are time-consuming. It can occur in particular in the case of workpiece forms that frequently change and for which a new determination of the optimal focal distance is required every time, that the particular preparation lasts longer than the actual processing of the workpiece. Furthermore, the selection of the optimal focal spot often proves to be difficult since the changes of the focal spot in the vicinity of the optimum are normally only small and can be recognized only with difficulty with the human eye. In order to nevertheless achieve an optimal processing result, it is necessary to reliably determine even extremely small changes in order to find the optimal parameters and pass them on to the laser lens. In particular, it can occur that the quality of a focal spot deteriorates only because, for example, the material located under it has an inhomogeneity although the change of the focal distance should actually have had the result of improving the quality of the focal spot. It can occur in such instances that only a local but not, however, a global minimum of the focal spot diameter is found and that the corresponding sub-optimal parameters are used in the later processing.
The problem of the invention is accordingly to make available a process and an apparatus for avoiding the disadvantages present in the state of the art, in particular the great expenditure of time in the determining of the optimal focal distance and the uncertainty in the selection of the optimal focal spot associated with an operator.